I. Field of the Invention
The present invention generally relates to communication systems, and, more particularly, to a method and apparatus for guard interval detection in a multi-carrier receiver.
II. Background of the Invention
In digital communication or digital broadcasting systems, information signals are transmitted in the form of electromagnetic waves from a transmitter, through a transmission channel, to a receiver. The transmitted information signals usually suffer from distortion on account of unideal channel effects such as multipath propagation or multipath fading. Recently, Orthogonal Frequency Division Multiplexing (OFDM) techniques have received a great deal of interest due to their effective transmission capability when dealing with various types of channel impairment, such as multipath fading and impulsive noise. Thus, OFDM has become the mainstream technology in the field of wireline communications, wireless communications and digital terrestrial television broadcasting. For instance, OFDM has been widely applied to ADSL (Asymmetrical Digital Subscriber Line), VDSL (Very-high-speed Digital Subscriber Line), HomePlug, DAB (Digital Audio Broadcasting), IEEE 802.11 a/g/n, 802.16a, HiperLAN/2, HiperMAN, DSRC (Dedicated Short Range Communications), UWB (Ultra Wideband), ISDB-T (Integrated Services Digital Broadcasting—Terrestrial), DVB-T (Digital Video Broadcasting—Terrestrial), DVB-H (Digital Video Broadcasting—Handheld) and so on.
In a DVB-T/DVB-H system, OFDM signals further include a cyclic prefix guard interval to each active symbol in order to reduce inter-symbol interference (ISI) resulting from multipath effects. Referring to FIG. 1, a data structure diagram of an active symbol 10 in combination with the associated guard interval 12 is illustrated schematically. Specifically, the guard interval 12 is the end portion 10A of the active symbol that is repeated before the current active symbol 10. The DVB-T standard defines two available modes of operation, each of which has a specific symbol size, which in turn defines the size of the Fast Fourier Transform (FFT) in a DVB-T receiver, namely 2K mode and 8K mode. In each of these carrier modes, there are four available guard interval lengths, namely ¼, ⅛, 1/16 and 1/32, of the respective FFT symbol duration, that is the period of the active symbol 10 as depicted in FIG. 1. The guard interval length can be selected based upon the anticipated multipath delay spread and transmission environment.
FIG. 2 is a block diagram of a conventional DVB-T receiver 2 with OFDM. As shown in FIG. 2, the received signals are input to a demodulator 20, which mixes the received RF input signal down to baseband in-phase (I) and quadrature (Q) components and produces digital samples. The baseband I and Q components are input to a mode detector 21 and thereafter to a guard interval detector 22. The mode detector 21 is employed to determine whether the received signals are transmitted in either 2K mode or 8K mode, and then provide mode detection outputs to a window position block 24, a remove guard block 25 and an FFT block 26. The guard interval detector 22 is employed to determine the guard interval length, and provide corresponding guard length outputs to the remove guard block 25 and window position outputs to the window position block 24 respectively. In FIG. 2, the baseband I and Q components are also input to a synchronizer 23, which removes at least one of frequency and sampling rate offsets and inputs signals to the window position block 24. Following determination of the window position, the processed received signals are input to the guard removal block 24, the FFT block 26, an equalizer 27, a demapper 28, and an FEC block 29. The resulting data, such as MPEG encoded signals, are output to a demultiplexing device for ultimate display.
Note that the receiver 2 of FIG. 2 is provided with the remove guard block 25 so as to remove these repetitive guard intervals and thus extract the active symbols for the FFT block 26 to allow the latter blocks to operate accurately. The remove guard block 25 performs such function according to the guard length outputs generated by the guard interval detector 22. If the guard interval length cannot be detected and removed accurately, the active symbols are so extracted inaccurately that the demodulation performance can be seriously degraded. Therefore, the guard interval detector 22 must include a mechanism for determining the length of the guard interval accurately. The chosen mechanism must be robust enough to deal with noisy signals, and with signals that suffer from multipath and co-channel interference.
The conventional guard interval detection mechanism uses a threshold value to detect the shape of the peak in a correlation function. However, an ideal threshold value is difficult to select. If the threshold value it too high, some of the points of potential interest will be missed, while, if the threshold value is too low, too many points will be examined, possibly leading to an inaccurate estimate of the guard interval length. Though it may be possible to set an initial threshold value in the guard interval detector, a further algorithm should be provided for adjusting the threshold value dynamically until a stable guard interval length can be obtained. Such algorithms, however, make the hardware architecture more complicated.
Therefore, a need exists for a method and apparatus for a multi-carrier receiver with guard interval length detection that can be easily implemented and overcomes the disadvantages of the conventional ones. The present disclosure provides such a multi-carrier receiver method and apparatus.
It is therefore an object of the present invention to provide a guard interval detector in a multi-carrier receiver that is robust enough to deal with noisy signals, and with signals that suffer from multipath and co-channel interference.
It is another object of the present invention to provide a guard interval detector in a multi-carrier receiver that can be employed to detect guard interval lengths of the received information signals accurately.
It is further object of the present invention to provide guard interval detector in a multi-carrier receiver that can be easily implemented without complicated circuit design.